I'll chime in on this.
No I don't think there should be any difference between buying a fully automatic firearm and a semi-automatic firearm (or any other type of firearm for that matter).
I have several reasons for believing this. The first has to do with the "shall not be infringed" part of the second amendment. I take the constitution as it is written, in other words if there was suppose to be an exception to the right to bear arms it would have been written in (such as the first amendment exception to inciting riots).
The next reason is the reason the second amendment was written in there. It's there so we could overthrow the government should they turn tyrannical. This would seem to imply the founding fathers meant for the citizens to have equal firepower to the government (they also assumed a small military force, but I shall not diverge).
A third reason I have is the fact that law abiding citizens aren't going to use a fully automatic weapon for any unlawful purpose. Criminals do that. Criminals are also the ones who can get fully automatic weapons without much trouble. Making it easier for a regular citizen to get a full auto won't increase the supply of such guns to criminals since they have means of obtaining them illegally anyways.
I find no justifiable reason to restrict gun ownership period. Gun control doesn't work for the purpose of preventing crime. Why should the majority of people have their rights restricted because a few people will use a machine gun in a criminal manner?